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    Subject[PATCH v2 26/52] x86, hpet: Fix CPU hotplug callback registration
    Date
    Subsystems that want to register CPU hotplug callbacks, as well as perform
    initialization for the CPUs that are already online, often do it as shown
    below:

    get_online_cpus();

    for_each_online_cpu(cpu)
    init_cpu(cpu);

    register_cpu_notifier(&foobar_cpu_notifier);

    put_online_cpus();

    This is wrong, since it is prone to ABBA deadlocks involving the
    cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
    with CPU hotplug operations).

    Instead, the correct and race-free way of performing the callback
    registration is:

    cpu_notifier_register_begin();

    for_each_online_cpu(cpu)
    init_cpu(cpu);

    /* Note the use of the double underscored version of the API */
    __register_cpu_notifier(&foobar_cpu_notifier);

    cpu_notifier_register_done();


    Fix the hpet code in x86 by using this latter form of callback registration.

    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: x86@kernel.org
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    arch/x86/kernel/hpet.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
    index da85a8e..d89382b 100644
    --- a/arch/x86/kernel/hpet.c
    +++ b/arch/x86/kernel/hpet.c
    @@ -943,12 +943,14 @@ static __init int hpet_late_init(void)
    if (boot_cpu_has(X86_FEATURE_ARAT))
    return 0;

    + cpu_notifier_register_begin();
    for_each_online_cpu(cpu) {
    hpet_cpuhp_notify(NULL, CPU_ONLINE, (void *)(long)cpu);
    }

    /* This notifier should be called after workqueue is ready */
    - hotcpu_notifier(hpet_cpuhp_notify, -20);
    + __hotcpu_notifier(hpet_cpuhp_notify, -20);
    + cpu_notifier_register_done();

    return 0;
    }


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